drivers: entropy: nrf5: add docs and a DSB in get_entropy_isr

Add a DSB before doing WFE in get_entropy_isr, to ensure
the memory transactions are complete.

Add a note clarifying a dependency for the existing solution
(dependency is satisfied by ARCH code but is good to state
clearly).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>